Diastereoselective ruthenium-catalysed cycloisomerisation of diallyllactones: preparation of exo-methylene spirolactones

Diallyllactones (obtained from cyclic anhydrides via a double allylation reaction promoted by titanium tetrachloride) were cycloisomerised using 5 mol% of cyclooctadienyl ruthenium dichloride in ethanol providing the corresponding exomethylene spirolactones in good yields, with moderated to good diastereomeric excess.